As a solar cell used for photovoltaic power generation, a solar cell composed of single crystal silicon, polycrystal silicon, amorphous silicon, or a compound such as cadmium telluride or copper indium selenide is a main research and development target, and part thereof is put in practical use. However, in order to widely expand use of these solar cells to an application such as a household power supply, these solar cells have problems such as difficulty in manufacturing the cells at a low cost or securing a raw material, and also a long energy payback time. These problems are required to be overcome. Meanwhile, many proposals have been made for solar cells produced by using an organic material for the purpose of increasing an area of a light-condensing part and providing the cells at a low price. However, the solar cells that have been applied so far have generally had a low conversion efficiency and a poor durability in many cases.
Under such a situation, a wet photoelectric conversion element that applies, as a working electrode, a titanium dioxide porous thin film spectrally sensitized with a ruthenium complex dye and a solar cell, and a material for preparing the same and a manufacturing technology therefor have been proposed (see, for example, Patent Literatures 1 and 2, and Non-Patent Literature 1). A first advantage of the wet photoelectric conversion elements described in these Literatures is to allow provision of the photoelectric conversion element at a low price because a less expensive oxide semiconductor such as titanium dioxide can be used without purification to a high purity. A second advantage of these wet photoelectric conversion elements is to absorb light in almost all wavelength regions of visible light and allow conversion of light into electricity because the dye to be used can absorb light in a wide-ranged wavelength band.
As a metal complex dye to be used for the photoelectric conversion element, N719, Z907, J2 or the like has been developed so far. A photoelectric conversion element prepared by using N719 shows a high photoelectric conversion efficiency initially. However, a decrease in conversion efficiency after use is large, and the cell has a problem of durability. Whereas, a photoelectric conversion element prepared by using Z907 has a small decrease in conversion efficiency after use. However, Z907 has a lower initial value per se of photoelectric conversion efficiency, as compared with N719.
Furthermore, a photoelectric conversion element containing semiconductor fine particles sensitized with a metal complex dye having a specific structure has been proposed (see, for example, Patent Literature 3). However, even the photoelectric conversion element described in Patent Literature 3 is not satisfactory in view of durability. J2 has also been developed as a dye having a high initial conversion efficiency (Patent Literature 4). However, the photoelectric conversion element is not satisfactory even with the initial conversion efficiency and durability of J2.
Consequently, a need is arisen for a dye that is excellent in the photoelectric conversion characteristics such as the conversion efficiency, and durability with a small decrease in the photoelectric conversion characteristics even after use over a long period of time, a photoelectric conversion element prepared by using the same as a sensitizing dye, and a dye-sensitized solar cell composed of such a photoelectric conversion element.